Nvim wasn't exiting cleanly in some job tests due to errors.
This can't be noticed until the next commit, which will perform a refactoring to
selectively process K_EVENT, so the `qa!` command executed at the end of each
test blocks forever if there are errors which require the user to press ENTER(in
that state Nvim no longer will process events).
Use save_tv_as_string(), same as vimL system(). This also makes
jobsend() more liberal in what it can accept. For example,
`jobsend(j, 123)` is now valid.
Closes#1176
Factor out string_to_list() from f_system()'s implementation
and use that to set job_data. This has the technical advantage of
preserving NULs, and may be more convenient for users.
Required for #1176.
